Here's a handy list of incentives to help you think through your your referral program strategy. We'll add some brief comments to help you think this through.
Single sided - This is an incentive for the sharer with no incentive for their friend (the referred visitor).
Two-sided (a.k.a. double sided) - This type of program has a referral incentive for the sharer and a conversion incentive for the visitor.
Loyalty points - If you have an active loyalty program, you can offer points to users who refer friends.
Cash - Particularly useful if your company has a low Customer Lifetime Value (LTV).
Store Credit - Particularly useful if you company has a high Customer Lifetime Value (LTV) like subscription businesses or where your customers come back and buy often.
Coupons - The cousin of store credit.
Gift - Can be used if you offer physical premiums like tote bags or have partnerships with third parties who want to give your customers a free trial of their product or service.
Donation to Charity - Your company can donate to a charity for every friend your customers refer. A great branding strategy.
Access to special content - Really great for companies that offer white papers or other tent pole content pieces.
Access to special features - Give customers who refer friends an upgrade. Great for SAAS and web-based apps like games.
Special events - This can include virtual events or on-the-ground galas.